Question: If in a deal on credit, the buyer says (to the seller), I will pay whenever I have the money; is this type of dealing correct?
The Grand Ayatollah Khamenei: Considering that in a deal on a credit, the time of paying the price must be known, and there is no probability of increase or decrease in it; the assumption of the question for such a deal is void (Batel). But if the seller is satisfied for the customer to use the sold goods, there is no problem for the customer to use it.
Source: khamenei.ir